Environmental Bright Light Exposure, Depression Symptoms, and Sleep Regularity

This cross-sectional study examines the associations between bright light therapy, sleep regularity, and depression symptoms among adults in the US.


Introduction
Bright light therapy (BLT) may treat depression symptoms, 1 but how light exerts mood-boosting effects is still under investigation.Here, we evaluate sleep regularity in the association between bright light and depression symptoms.

Methods
This cross-sectional study of 2011-2014 National Health and Nutrition Examination Survey (NHANES) data, representative of the noninstitutionalized US population, included nonpregnant participants 18 years or older with valid light and actigraphy (wrist-worn GT3X+; ActiGraph) for bright light (time above lux threshold [TALT 1000 ]) and sleep regularity index (SRI) 2 measures (eMethods in Supplement 1) and follows STROBE guidelines.Depression symptoms were measured with the Patient Health Questionnaire-9 (PHQ-9; scores range from 0 to 30, with higher scores indicating greater frequency of depression symptoms).This analysis used publicly available, deidentified data from NHANES, which was approved by the Ethics Review Board of the Centers for Disease Control and Prevention.All participants provided written informed consent.c Cannot be specified further.

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Open Access.This analysis was conducted between March 2023 and May 2024.Associations between TALT 1000 , SRI, and PHQ-9 score were tested in weighted linear, quasi-Poisson, and logistic regression models adjusted for age, sex, race and ethnicity, season, employment, physical activity, sleep duration, body mass index (BMI), and serum cotinine (eMethods in Supplement 1).Sex differences were tested with weighted t tests.In exploratory analyses, associations between TALT 1000 and vitamin D, vitamin D and SRI, and TALT 1000 and SRI with vitamin D as a potential mediator were tested in weighted linear regression models.P < .05 was considered statistically significant.Analyses were performed in R, version 4.1.1(R Foundation).

Results
There

Discussion
In this study, greater sleep regularity partly explained the association between greater bright light exposure and lower depression symptoms.More vitamin D was associated with more regular sleep timing but not depression.Our findings align with prior population-based research, supporting reduced depression among those with greater daytime light exposure. 3Bright light therapy can be effective at improving numerous mood outcomes, 4 possibly through effects on sleep and the circadian system. 5eep regularity may be an important modifier of BLT 2 and relate to chronotype and phase angle of entrainment. 6However, sleep regularity is not often considered in BLT.The phase-shifting effect of light may be limited in people with irregular sleep schedules, requiring a larger dose or altered timing of light exposure.Irregular sleepers may benefit from a light intervention. 2is study has strengths and limitations.It used a nationally representative sample of adult participants with objective measures of individual light exposure, actigraphy, and vitamin D. This is a cross-sectional analysis, so causality cannot be ascertained, and associations may be bidirectional.
Although we are unable to rule out that mood may also influence time spent outdoors and/or bright light avoidance, our results support the need for further prospective analyses to test the causality of these factors.Future studies of BLT should consider the role of sleep regularity.

Table 1 .
Variable Mean Values by Total Sample and by Sex a a Sample-weighted values are provided for mean (SD) and number (%), in addition to number of participants for categorical variables.b Sex differences were tested with 2-sample t tests or χ 2 tests.

Table 2 )
. In exploratory analyses, more vitamin D was associated with greater TALT 1000 and more regular sleep but not depression symptoms.